Fluorine, the essential element for fluorochemicals and fluoropolymers, is currently derived from fluorospar, a mineral which is a crystalline form of calcium fluoride. Reserves of fluorospar are rapidly being depleted. Furthermore, the United States currently imports about 90% of its supply.
An important reserve of fluorine is contained in fluoroapatite, CaF293.Ca.sub.3 (PO.sub.4).sub.2, a mineral which is used for the manufacture of phosphoric acid. This mineral constitutes a reserve which is about four times greater than the proven reserves of fluorospar. During the manufacture of phosphoric acid most of the fluorine is removed as fluosilicic acid (FSA). There is some demand for FSA for fluoridating drinking water, for the manufacture of cryolite and aluminum fluoride. However, since this demand is small, most of the fluorine produced during phosphoric acid manufacture is sent to a waste-water pond. This can cause a fluorine pollution problem.
Over the years numerous processes, several of which are described below, have been developed to recover the fluorine from phosphate minerals. The United States Bureau of Mines (Chem. Abst., 75:23270, 1971) has shown how waste fluosilicic acid can be converted to an acid-grade fluorospar (CaF.sub.2). A first step involves treating the FSA with ammonia to precipitate silica, which is removed by filtration, and form NH.sub.4 F. In a second step ammonium fluoride is treated with lime to form CaF.sub.2.
U.S. Pat. No. 3,549,317 discloses a process for the preparation of a silica filler and a calcium fluoride from FSA or sodium fluosilicate in water. The fluosilicate is reacted with calcium carbonate at a temperature above 50.degree. C. until evolution of CO.sub.2 stops, at which point calcium fluoride and a silica material is formed. The silica is dissolved in sodium hydroxide at temperatures of 50.degree. C. to the mixture's boiling point and the calcium fluoride is then removed by precipitation.
U.S. Pat. No. 4,264,563 describes a process for the preparation of a calcium fluoride-containing solid in which a fluorosilicate-containing material is added to a heated suspension of lime in water.
U.S. Pat. No. 5,531,975 describes a process for reacting phosphate rock and PSA to produce a slurry comprising phosphoric acid, calcium fluoride, silicon dioxide and undigested phosphate rock.
There is still a need for an efficient commercial process for preparing useful fluorine containing materials such as calcium fluoride from FSA.